This invention relates to vacuum processing of a workpiece and, more particularly, to systems for controlling a gap between the tip of a vacuum envelope apparatus and a surface of a workpiece in a localized vacuum processing system.
Particle beam systems, such as ion implanters, electron beam lithography systems and ion beam lithography systems, require evacuation of the region along the path between the particle beam source and the workpiece being treated. Various types of vacuum processing chambers have been used. In the simplest, workpieces are loaded either singly or in a batch into a work chamber containing the particle beam source. After loading, the chamber is sealed and is vacuum pumped until the desired level of vacuum is achieved. This approach is relatively slow, since the entire volume of the work chamber must be vacuum pumped each time a new batch of workpieces is introduced. Additional problems include outgassing of absorbed gases and introduction of contaminants. In many processes, for example, commercial semiconductor processing, throughput is of utmost importance. To increase processing speed, air locks have been used for introducing the workpiece into the processing chamber. The workpiece is introduced into a small volume which is then sealed and vacuum pumped. When the desired level of vacuum is reached, a door or valve connecting the air lock to the processing chamber is opened. The vacuum pumping time is reduced by reducing the volume to be pumped.
Certain particle beam processes, such as electron beam lithography, require movement of the workpiece during processing. In electron beam lithography, microminiature patterns of extremely high accuracy are exposed on the workpiece. To achieve high accuracy, the deflection field of the electron beam is limited to an area much smaller than the area of the workpiece. Typically, the deflection field in an electron beam system is one or a few millimeters, while the workpiece, usually a semiconductor wafer or a mask plate, can be up to several inches in dimension. Thus, precise positioning of the workpiece is required. In the past, the workpiece has been positioned by a system of drive motors external to the vacuum region and a mechanical linkage has been used to transmit the motion into the vacuum through bellows or rotary vacuum seals. Such systems have been large, complex and relatively unreliable. In spite of the size and complexity of such positioning systems, the particle beam is scanned over a very small localized region of the workpiece surface at any given time.
Apparatus for vacuum processing in a localized region on the surface of a workpiece is disclosed in pending application Ser. No. 435,179 filed concurrently herewith and assigned to the assignee of the present invention. The vacuum processing apparatus includes an envelope which defines an internal vacuum processing zone. The tip of the envelope is positioned just above the surface of the workpiece and is spaced from the workpiece by a preselected gap. The tip of the envelope together with the workpiece forms a non-contacting graded vacuum seal between the internal vacuum processing zone and the ambient environment. The envelope tip typically has the general shape of a cone and provides a vacuum zone on the surface of the workpiece which is small in comparison with the size of the workpiece.
In order to treat the entire surface of the workpiece, relative movement between the envelope and the workpiece is required. Typically, the workpiece, is mounted on a stage which is laterally movable, with respect to the tip of the envelope. To achieve rapid processing of workpieces in systems such as electron beam lithography systems, stage speeds of between 1 and 10 cm/second are typically employed. During this movement, the gap between the tip of the envelope and the workpiece must be dynamically controlled within specified limits. If the gap becomes too large, the vacuum in the vacuum zone is reduced, and processing is interrupted until the required vacuum level can again be achieved. Electron beam columns used in lithography systems must be operated in the mid to high vacuum range, and, typically, inadequate space is available to provide high conductance vacuum pumping. In such cases, the gap must be relatively small. When high conductance vacuum pumping can be provided or lower pressure can be tolerated by the process, a larger gap is permitted. Conversely, the gap cannot become so small as to risk contact between the tip of the envelope and the workpiece. A workpiece such as a semiconductor wafer is extremely fragile and can be permanently damaged or broken by such contact.
While workpieces such as semiconductor wafers are in general flat and planar, they are subject to thickness variations, surface irregularities and warping, all of which can cause gap variations during the lateral movement of the wafer. Furthermore, the stage system, which provides the lateral movement of the wafer with respect to the tip of the envelope, is subject to certain imperfections and tolerances which can cause the gap to vary as the wafer is moved. These factors make it necessary to dynamically control the gap between the tip of the envelope and the workpiece during processing.
It is, therefore, an object of the present invention to provide apparatus for controlling the gap between a localized vacuum processing envelope and a surface of a workpiece.
It is another object of the present invention to provide parallel alignment between a generally planar workpiece and the generally planar tip of a localized vacuum processing envelope.
It is yet another object of the present invention to provide a novel method for sensing the gap between a localized vacuum envelope and a workpiece.
It is still another object of the present invention to provide apparatus for controlling the gap between the tip of a localized vacuum processing envelope apparatus and a workpiece so as to prevent degradation of the vacuum level in the envelope apparatus and to prevent contact between the envelope apparatus and the workpiece.